Copy-proof recording medium and device for adding copy-proof interference signal

Abstract

A copy-proof interference signal or signals is/are added to the composite color video signal to be recorded on a recording medium in the vicinity of the horizontal synchronous signal. The interference signal comprises a pulse having a narrow width compared to the width of the horizontal synchronous signal pulse. A negative going pulse may also be added to the front porch of the horizontal synchronous signal so as to compensate for area-decrease of the horizontal synchronous signal due to the addition of an interference signal. The interference signal may be added to the videl signal region which is close to the leading edge of the horizontal synchronous signal so that the interference signal does not cause the occurrence of noises or distortion in a reproduced picture. When the interference signal-added composite color video signal is directly applied to a monitor TV, satisfactory color pictures can be obtained as usual. However, when the signal reproduced from the recording medium is once recorded by a video tape recorder (VTR), only nonpractical color pictures can be obtained with the signal from the VTR because the phase-restoring operation of chromatic signal on reproduction is disturbed by the interference signal.

What is claimed is: 
     
       1. A copy-proof recording medium carrying a composite color video signal to which a copy-proof interference signal has been added so that said interference signal will cause a synchronous separation circuit of a video signal recording apparatus to produce a false horizontal synchronous signal when said composite color video signal of said medium is recorded by said video recording apparatus, said interference signal having a pulse which is synchronous with a horizontal synchronous pulse of said composite color video signal, said pulse of said interference signal having a width considerably smaller than the pulse width of the horizontal synchronous signal of said composite color video signal, said interference signal being added in at least one particular place, within the horizontal synchronous signal, at a position which precedes a midway point of the horizontal synchronous signal, said interference signal being a positive-going pulse which rises from the level of the tip of the horizontal synchronous signal to a level equal to or greater than the pedestal level, the position, the amplitude and the width of said pulse of said interference signal being maintained constant. 
     
     
       2. A copy-proof recording medium as claimed in claim 1, wherein said composite color video signal comprises a negative going pulse in the vicinity of the leading edge of said horizontal synchronous signal for compensating for decrease in area defined by a single pulse of said horizontal synchronous signal, which decrease is caused by addition of said interference signal. 
     
     
       3. A copy-proof recording medium as claimed in claim 2, wherein said negative going pulse is added to the front porch of said horizontal synchronous signal pulse. 
     
     
       4. A copy-proof recording medium as claimed in claim 1, wherein said interference signal is added to each pulse of said horizontal synchronous signal. 
     
     
       5. A copy-proof recording medium as claimed in claim 1, wherein said interference signal is added to some pulses of said horizontal synchronous signal. 
     
     
       6. A device for adding at least one copy-proof interference signal to a composite color video signal so that said interference signal will cause a synchronous separation circuit of a video signal recording apparatus to produce a false horizontal synchronous signal when said composite color video signal is recorded by said video recording apparatus, comprising: first means responsive to the horizontal synchronous signal of said composite color video signal for producing a pulse train which is synchronous with a horizontal synchronous pulse of said composite color video signal; second means for defining the width of pulses of said pulse train so that said pulse has a width considerably smaller than the pulse width of the horizontal synchronous signal of said composite color video signal; and third means for adding said pulse to said composite color video signal such that said interference signal is added in at least one particular place, within the horizontal synchronous signal, at a position which precedes a midway point of the horizontal synchronous signal, said interference signal being a positive-going pulse which rises from the level of the tip of the horizontal synchronous signal to a level equal to or greater than the pedestal level, the position, the amplitude and the width of said first pulse of said interference signal being maintained constant. 
     
     
       7. A device as claimed in claim 6, wherein said third means comprises a mixer-amplifier. 
     
     
       8. A device as claimed in claim 6, wherein said first means comprises a first monostable multivibrator responsive to a pulse synchronous with said horizontal synchronous signal. 
     
     
       9. A device as claimed in claim 8, wherein said first means comprises a second monostable multivibrator responsive to an output pulse of said first monostable multivibrator. 
     
     
       10. A device as claimed in claim 9, further comprising a level adjuster for adjusting the amplitude of the output pulse of said second monostable multivibrator. 
     
     
       11. A device as claimed in claim 9, further comprising a third monostable multivibrator responsive to a pulse synchronous with said horizontal synchronous signal and a fourth monostable multivibrator responsive to the output pulse of said third monostable multivibrator. 
     
     
       12. A device as claimed in claim 11, further comprising a first level adjuster for adjusting the amplitude of the output pulse of said second monostable multivibrator and a second level adjuster for adjusting the amplitude of the output pulse of said fourth monostable multivibrator.